Known dual hardness steel armor (DHA) consists of a relatively thin front layer of hard steel facing the expected impact direction of a projectile and a relatively thick back layer of softer steel, and it is known to be highly effective against firearm projectiles due to the differences in the hardness of its layers.
U.S. Pat. No. 5,749,140 discloses a method of producing armor of the kind described above, which involves heat treatment, in which: “ . . . each steel composition would be melted and hot rolled to an intermediate slab thickness” and which involves “peripheral welding to form packs on the front and rear slabs, possibly, but not necessarily, evacuating and hermetically sealing the slabs, thereafter, roll bonding to the desired plate thickness and subsequently heat treating by austenitizing, quenching, and tempering as necessary.”
U.S. Pat. No. 5,483,864 discloses another method for producing an armor of the above type, which involves creating a first metal alloy layer, letting it cool down to a semi-liquid state, and then spraying on top of it molten metal alloy to form a second layer. Both layers are then cooled down to form the dual hardness armor.